diff --git a/hpb/extension.h b/hpb/extension.h
index 111bbc712d..ae8d23b19a 100644
--- a/hpb/extension.h
+++ b/hpb/extension.h
@@ -9,15 +9,19 @@
 #define GOOGLE_PROTOBUF_HPB_EXTENSION_H__
 
 #include <cstdint>
+#include <type_traits>
 #include <vector>
 
 #include "absl/base/attributes.h"
+#include "absl/status/statusor.h"
 #include "google/protobuf/hpb/backend/upb/interop.h"
 #include "google/protobuf/hpb/internal/message_lock.h"
 #include "google/protobuf/hpb/internal/template_help.h"
 #include "google/protobuf/hpb/ptr.h"
 #include "google/protobuf/hpb/status.h"
 #include "upb/mem/arena.hpp"
+#include "upb/message/accessors.h"
+#include "upb/message/array.h"
 #include "upb/mini_table/extension.h"
 #include "upb/mini_table/extension_registry.h"
 
@@ -34,6 +38,41 @@ absl::Status SetExtension(upb_Message* message, upb_Arena* message_arena,
                           const upb_MiniTableExtension* ext,
                           const upb_Message* extension);
 
+/**
+ * Trait that maps upb extension types to the corresponding
+ * return value: ubp_MessageValue.
+ *
+ * All partial specializations must have:
+ * - DefaultType: the type of the default value.
+ * - ReturnType: the type of the return value.
+ * - kGetter: the corresponding upb_MessageValue upb_Message_GetExtension* func
+ */
+template <typename T, typename = void>
+struct UpbExtensionTrait;
+
+template <>
+struct UpbExtensionTrait<int32_t> {
+  using DefaultType = int32_t;
+  using ReturnType = int32_t;
+  static constexpr auto kGetter = upb_Message_GetExtensionInt32;
+};
+
+template <>
+struct UpbExtensionTrait<int64_t> {
+  using DefaultType = int64_t;
+  using ReturnType = int64_t;
+  static constexpr auto kGetter = upb_Message_GetExtensionInt64;
+};
+
+// TODO: b/375460289 - flesh out non-promotional msg support that does
+// not return an error if missing but the default msg
+template <typename T>
+struct UpbExtensionTrait<T> {
+  using DefaultType = int;
+  using ReturnType = int;
+  using DefaultFuncType = void (*)();
+};
+
 // -------------------------------------------------------------------
 // ExtensionIdentifier
 // This is the type of actual extension objects.  E.g. if you have:
@@ -54,14 +93,24 @@ class ExtensionIdentifier {
   }
 
  private:
-  constexpr explicit ExtensionIdentifier(const upb_MiniTableExtension* mte)
-      : mini_table_ext_(mte) {}
+  constexpr explicit ExtensionIdentifier(
+      const upb_MiniTableExtension* mte,
+      typename UpbExtensionTrait<ExtensionType>::DefaultType val)
+      : mini_table_ext_(mte), default_val_(val) {}
+
   constexpr uint32_t number() const {
     return upb_MiniTableExtension_Number(mini_table_ext_);
   }
 
-  friend struct PrivateAccess;
   const upb_MiniTableExtension* mini_table_ext_;
+
+  typename UpbExtensionTrait<ExtensionType>::ReturnType default_value() const {
+    return default_val_;
+  }
+
+  typename UpbExtensionTrait<ExtensionType>::DefaultType default_val_;
+
+  friend struct PrivateAccess;
 };
 
 upb_ExtensionRegistry* GetUpbExtensions(
@@ -216,10 +265,19 @@ absl::StatusOr<Ptr<const Extension>> GetExtension(
 
 template <typename T, typename Extendee, typename Extension,
           typename = hpb::internal::EnableIfHpbClass<T>>
-absl::StatusOr<Ptr<const Extension>> GetExtension(
+decltype(auto) GetExtension(
     const T* message,
-    const ::hpb::internal::ExtensionIdentifier<Extendee, Extension>& id) {
-  return GetExtension(Ptr(message), id);
+    const hpb::internal::ExtensionIdentifier<Extendee, Extension>& id) {
+  if constexpr (std::is_integral_v<Extension>) {
+    auto default_val = hpb::internal::PrivateAccess::GetDefaultValue(id);
+    absl::StatusOr<Extension> res =
+        hpb::internal::UpbExtensionTrait<Extension>::kGetter(
+            hpb::interop::upb::GetMessage(message), id.mini_table_ext(),
+            default_val);
+    return res;
+  } else {
+    return GetExtension(Ptr(message), id);
+  }
 }
 
 template <typename T, typename Extension>
diff --git a/hpb/internal/internal.h b/hpb/internal/internal.h
index beedcf7c78..2634c3cf48 100644
--- a/hpb/internal/internal.h
+++ b/hpb/internal/internal.h
@@ -47,6 +47,11 @@ struct PrivateAccess {
   static constexpr uint32_t GetExtensionNumber(const ExtensionId& id) {
     return id.number();
   }
+
+  template <typename ExtensionId>
+  static decltype(auto) GetDefaultValue(const ExtensionId& id) {
+    return id.default_value();
+  }
 };
 
 }  // namespace hpb::internal
diff --git a/hpb_generator/gen_extensions.cc b/hpb_generator/gen_extensions.cc
index 959da3db3a..cf9650d315 100644
--- a/hpb_generator/gen_extensions.cc
+++ b/hpb_generator/gen_extensions.cc
@@ -8,10 +8,13 @@
 #include "google/protobuf/compiler/hpb/gen_extensions.h"
 
 #include <string>
+#include <vector>
 
 #include "absl/strings/str_cat.h"
 #include "google/protobuf/compiler/hpb/context.h"
+#include "google/protobuf/compiler/hpb/gen_utils.h"
 #include "google/protobuf/compiler/hpb/names.h"
+#include "google/protobuf/descriptor.h"
 #include "upb_generator/c/names.h"
 
 namespace google::protobuf::hpb_generator {
@@ -73,6 +76,7 @@ void WriteExtensionIdentifier(const protobuf::FieldDescriptor* ext,
       {{"containing_type_name", ContainingTypeName(ext)},
        {"mini_table_name", mini_table_name},
        {"ext_name", ext->name()},
+       {"default_value", DefaultValue(ext)},
        {"ext_type", CppTypeParameterName(ext)},
        {"class_prefix", class_prefix}},
       R"cc(
@@ -82,7 +86,7 @@ void WriteExtensionIdentifier(const protobuf::FieldDescriptor* ext,
                 ::hpb::internal::PrivateAccess::InvokeConstructor<
                     ::hpb::internal::ExtensionIdentifier<$containing_type_name$,
                                                          $ext_type$>>(
-                    &$mini_table_name$);
+                    &$mini_table_name$, $default_value$);
       )cc");
 }
 
diff --git a/hpb_generator/gen_utils.cc b/hpb_generator/gen_utils.cc
index f89afc99e2..d0238d0104 100644
--- a/hpb_generator/gen_utils.cc
+++ b/hpb_generator/gen_utils.cc
@@ -11,8 +11,11 @@
 #include <string>
 #include <vector>
 
+#include "absl/log/absl_log.h"
 #include "absl/strings/ascii.h"
+#include "absl/strings/str_cat.h"
 #include "absl/strings/string_view.h"
+#include "google/protobuf/descriptor.h"
 
 namespace google::protobuf::hpb_generator {
 
@@ -127,5 +130,20 @@ std::string ToCamelCase(const absl::string_view input, bool lower_first) {
   return result;
 }
 
+std::string DefaultValue(const FieldDescriptor* field) {
+  switch (field->cpp_type()) {
+    case FieldDescriptor::CPPTYPE_INT32:
+      return absl::StrCat(field->default_value_int32());
+    case FieldDescriptor::CPPTYPE_INT64:
+      return absl::StrCat(field->default_value_int64());
+    default:
+      // TODO: b/375460289 - implement rest of scalars + msg
+      ABSL_LOG(WARNING) << "Unsupported default value type (in-progress): <"
+                        << field->cpp_type_name()
+                        << "> For field: " << field->full_name();
+      return "-1";
+  }
+}
+
 }  // namespace protobuf
 }  // namespace google::hpb_generator
diff --git a/hpb_generator/gen_utils.h b/hpb_generator/gen_utils.h
index a83500c719..154824776e 100644
--- a/hpb_generator/gen_utils.h
+++ b/hpb_generator/gen_utils.h
@@ -41,6 +41,8 @@ std::vector<const protobuf::FieldDescriptor*> FieldNumberOrder(
 
 std::string ToCamelCase(absl::string_view input, bool lower_first);
 
+std::string DefaultValue(const FieldDescriptor* field);
+
 }  // namespace protobuf
 }  // namespace google::hpb_generator
 
diff --git a/hpb_generator/tests/test_extension.proto b/hpb_generator/tests/test_extension.proto
index f7758da7df..e8ff9d5329 100644
--- a/hpb_generator/tests/test_extension.proto
+++ b/hpb_generator/tests/test_extension.proto
@@ -17,3 +17,8 @@ import "google/protobuf/compiler/hpb/tests/test_model.proto";
 extend TestModel {
   optional ThemeExtension styling = 13001;
 }
+
+extend TestModel {
+  optional int32 int32_ext = 13002 [default = 644];
+  optional int64 int64_ext = 13003 [default = 2147483648];
+}
diff --git a/hpb_generator/tests/test_generated.cc b/hpb_generator/tests/test_generated.cc
index f9fb8132f7..23c0cfe0e0 100644
--- a/hpb_generator/tests/test_generated.cc
+++ b/hpb_generator/tests/test_generated.cc
@@ -5,6 +5,7 @@
 // license that can be found in the LICENSE file or at
 // https://developers.google.com/open-source/licenses/bsd
 
+#include <climits>
 #include <cstdint>
 #include <iterator>
 #include <limits>
@@ -54,6 +55,8 @@ using ::hpb_unittest::protos::TestModel_Category_NEWS;
 using ::hpb_unittest::protos::TestModel_Category_VIDEO;
 using ::hpb_unittest::protos::theme;
 using ::hpb_unittest::protos::ThemeExtension;
+using ::hpb_unittest::someotherpackage::protos::int32_ext;
+using ::hpb_unittest::someotherpackage::protos::int64_ext;
 using ::testing::ElementsAre;
 
 TEST(CppGeneratedCode, Constructor) { TestModel test_model; }
@@ -878,6 +881,21 @@ TEST(CppGeneratedCode, GetExtension) {
             hpb::GetExtension(&model, theme).value()->ext_name());
 }
 
+TEST(CppGeneratedCode, GetExtensionInt32WithDefault) {
+  TestModel model;
+  auto res = hpb::GetExtension(&model, int32_ext);
+  EXPECT_TRUE(res.ok());
+  EXPECT_EQ(*res, 644);
+}
+
+TEST(CppGeneratedCode, GetExtensionInt64WithDefault) {
+  TestModel model;
+  auto res = hpb::GetExtension(&model, int64_ext);
+  EXPECT_TRUE(res.ok());
+  int64_t expected = std::numeric_limits<int32_t>::max() + int64_t{1};
+  EXPECT_EQ(*res, expected);
+}
+
 TEST(CppGeneratedCode, GetExtensionOnMutableChild) {
   TestModel model;
   ThemeExtension extension1;